Separators of the kind under discussion commonly comprise a vertical column about which there are supported one or more helical troughs coaxially nested in the manner of a multi-start screw thread. The troughs are typically but not essentially of uniform pitch throughout their length. Each trough is provided with a pulp inlet at the upper end of the working portion of the trough, the inlet being connected by tubular means with a common header tank for spiral separators operating in parallel whereby a pulp in the tank may be fed to each inlet. When more than one trough is supported by the column, the respective pulp inlets of the troughs are commonly arranged as nearly as possible to be in a horizontal plane to facilitate simultaneous introduction of pulp to each helix. In the case of two troughs supported on a column the inlets are typically diametrically opposite each other and in the case of the three troughs are equiangularly spaced in a horizontal plane.
Each trough has a floor situated between an outer trough wall and an inner trough wall. In some separators the column may be, or may be a part of, the inner wall wall. In cross-section, with respect to the helix radial direction, the bottom working portion of the trough floor generally inclines upwardly from the inner wall or column to the outer wall. It will be understood that the trough floor at its radially innermost end curves upward to blend with the inner wall or column and at its radially outermost end curves radially upwards to blend with the outer wall.
In operation, pulp is fed from the header to the inlet of each trough. As the stream of pulp descends the trough, particles of higher specific gravity tend to segregate to the bottom of the stream and then slow through contact with the surface and gravitate radially inwardly while particles of lighter specific gravity tend to move radially outwards by virtue of centrifugal forces which overcome the inwards gravitational component. Splitters are arranged at variuus levels of each trough whereby each descending stream may be split into fractions and desired fractions are withdrawn at outlets associated with the splitters. The setting of the splitters requires supervision and frequent readjustment during use of the apparatus in order to maintain acceptable yields.
An object of the present invention is to provide a trough separator which in preferred embodiments is simpler to operate and produces a higher yield of desired fractions than those known in the prior art.